By drilling a hole larger than is needed and filling it with a two-part epoxy, a do-it yourself boat owner can protect balsa, foam or plywood cores from potential water intrusion when installing chimneys, solar vents, cleats, winches, inspection plates and more on the topsides of fiberglass boats.
Structure of Fiberglass Boat Tops
Most fiberglass boats are a sandwich of two fiberglass layers with a balsa wood, foam or plywood core. The core adds a lightweight strength, but even small amounts of water leaking though into the core layer can cause structural damage over time, so take precautions when drilling through the boat's topsides.
Before Drilling Through Cabin Roof, Locate Boat's Electrical Wires
Some boat manufacturers place their wiring in the gap between the exterior layer of fiberglass and the interior cabin liner. Before deciding to drill in to the cabin roof, it’s important to know what is sandwiched between the layers. If no manufacturer’s diagrams are available, see if there are removable panels anywhere, and failing that, note where lights and electrical equipment are relative to the boat’s electrical supply — both the shore power and the 12-volt system — then guess the most logical route between the two.
Measure Several Times Before Drilling Through Foam Core Fiberglass
Most boats have very little unused space. So while the planned location for a solar vent, for example, may be perfect from inside the cabin, on the exterior, it may be under the dodger or in the way of lines, foot traffic, or other topside gear or structures. Measure the placement both inside the cabin and outside several times, then draw the dimensions of the hole on both the inside and outside of the coach roof.
Drill a Test Hole in the Coach Roof
Once the hole is drawn, mark the center point, then with a very fine drill bit, drill through all layers of fiberglass and core. This test hole will answers several questions:
- Are the inside and outside measurements aligned? If not, how far out of alignment are they?
- How thick is the fiberglass and core in this location?
- What is the core material, and what is its condition? Wet, dry rotted, or sound?
Drill the Hole Larger than Needed
If the test hole shows that the measurements on the inside and outside line up pretty well, and there are no surprises, like bits of copper wire, or wet balsa wood in the drill shavings, it is time to proceed. For holes under one inch in diameter, drill or cut a hole about an eighth of an inch larger than needed. For holes larger than an inch in diameter, drill or cut a hole at least a quarter inch larger than needed. For example, if a quarter inch hole is needed, drill 3/8 inch hole, for a 2-inch hole, cut a 2-1/4 inch hole using a hole saw.
Fill the Hole with Epoxy
Cover the underside of the hole with a piece of waxed paper and enough layers of strong tape to support the weight of wet epoxy. If the hole is fairly large, reinforce the tape with a layer of cardboard. If the hole is larger than 3 inches, build a dam as explained below. Mix the epoxy according to the package directions, and fill the hole.
Use a Dam on Large Holes
To conserve epoxy on large holes like those used for solar vents, inspection hatches, or chimneys, use a cardboard ring as a dam so that the epoxy will make a donut against the core material, and the center of the hole will be empty. Allow the epoxy to cure according to the manufacturer’s directions.
Finally Drill the Finished Size Hole
Remove the tape and wax paper from the underside of the drilled hole, sand smooth and paint if necessary. Drill the required size hole into the epoxy filler, and complete the installation of the new product following the manufacturer’s instructions. Use marine sealant around any exposed fittings.
By creating an epoxy barrier between vulnerable core materials and the new instillation, there is far less chance water will seep into the core and cause structural damage.
Sheila Gaquin - As a public school teacher Sheila has worked in some remote and far flung places. She taught for nine years in a remote Eskimo village ...
